Wireless remote speaker systems for high fidelity stereo use are popular due to ease of movement to locations not readily served by conventional wired speakers. For true portability, these remote amplified speakers must be battery powered. Size and weight constraints limit the amphere-hour capacity of built-in rechargeable batteries, so that efficient use of the available power must be a primary concern. One important factor in prolonging battery life is turning the unit off when the audio signals are no longer being received. Leaving the units "on" inadvertently for extended periods of time, not only discharges the batteries, but such deep discharge is harmful to most secondary battery systems.
One type of automotive automatic turn-on and turn-off system is discloses in U.S. Pat. No. 4,453,264 issued June 5, 1984 to the inventor of the subject invention. The input signal is AC coupled to a differential amplifier stage which generates a ground referenced signal. The signal is AC coupled to a voltage comparator. The comparator in each channel is set to develop a positive going output pulse whenever the input exceeds a predetermined level. The output pulses which follow the input signal excursions are OR'ed by diodes and feed an integrator capacitor. The voltage across the integrator capacitor is monitored by a MOSFET switch which in turn, pulls in a power control relay. Typical transistor characteristics allow the device to turn on whenever the voltage across the integrator capacitor reaches a predetermined level. In the absence of any audio input signal, the charge on the capacitor dissipates slowly through a bleeder resistor so as to shut off the FET and drop out the relay after some delay. A problem with this system is that the turn-on sensitivity is linked directly to turn-off time delay. An additional problem with this type of technology is that human hearing being essentially logarithmic in sensitivity can easily detect very low audio levels in the existing speakers if they are used. The auxiliary, amplified speaker should switch on automatically at this low level and not at higher levels where a sudden turn-on could be very annoying. Random audio input line and power line noise may trigger the system to turn on. Reducing the turn-on sensitivity by increasing the comparator set point does render the system more noise immune, but increases the turn-on delay.
A second system is disclosed in an article titled Sound Operated Switch written by Michael Tooley and David Whitfield in Practical Electronics, May 1979 Edition. The article discloses a switch which turns on and off in response to an audio frequency signal. The incoming audio frequency signal is applied to an amplifier stage and then to a unity gain active full wave rectifier. The output excursion of the rectifier is applied to the comparator whose output is used to enable a conventional timer circuit, which is a 555 integrated timer circuit. The article stresses that once the input level has risen sufficiently to trigger the circuit and enable the timer, the output of the monostable will remain high regardless of any subsequent reduction of input level. The problem with this system is that it is not retriggerable during the time delay. The integrated timer circuit receives an input signal and produces an output signal for the predetermined time after which it goes low and waits for another input signal. This could cause blanking out of the speaker.
Battery operated, amplified remote speakers should not turn-on automatically because many stereo speaker pairs could be placed within a building and adjacent spaces and some of these areas would not be occupied at all times. Instead of all remote speakers automatically turning on simultaneously whenever the master transmitter is energized, it is preferable that individual speaker pairs be manually turned on as required.
There are other wireless remote speaker systems on the market which all suffer major drawbacks. They are generally either designed for line of sight use from the remote transmitter (IR LED types) or are not truly wireless, in that they utilize the AC power line for power and current carrier transmission of the audio signal. Being tied to the AC power line strictly limits the possible points of use, but existing battery powered remote speakers simply do not preform adequately mainly because of limited use time per charge.